Electrode materials with high surface conductivity

ABSTRACT

The present invention concerns electrode materials capable of redox reactions by electron and alkali-ion exchange with an electrolyte. The applications are in the field of primary (batteries) or secondary electrochemical generators, supercapacitors and light modulating systems of the electrochromic type.

The present application is a continuation of U.S. patent application Ser. No. 12/033,636, filed Feb. 19, 2008 (now U.S. Pat. No. 7,815,819), which is a divisional of U.S. patent application Ser. No. 11/266,339, filed Nov. 4, 2005 (now U.S. Pat. No. 7,344,659), which is a continuation of U.S. application Ser. No. 10/740,449 filed Dec. 22, 2003, (now U.S. Pat. No. 6,962,666), which is a divisional of U.S. application Ser. No. 10/175,794, filed Jun. 21, 2002 (now U.S. Pat. No. 6,855,273), which is a continuation of U.S. application Ser. No. 09/560,572, filed Apr. 28, 2000, now abandoned, which claims the benefit of CA 2,270,771, filed Apr. 30, 1999. The entire contents of which are hereby incorporated herein by reference.

FIELD OF INVENTION

The present invention concerns electrode materials capable of redox reactions by electron and alkali-ion exchange with an electrolyte. The applications are in the field of primary (batteries) or secondary electrochemical generators, supercapacitors and light modulating systems of the electrochromic type.

BACKGROUND OF THE INVENTION

Insertion compounds (hereinafter also referred to as electroactive materials or redox materials) are well known, and their operation is based on the exchange of alkali ions, in particular lithium ions, and valence electrons of at least one transition element, in order to keep the neutrality of the solid matrix. The partial or complete maintenance of the structural integrity of the material allows the reversibility of the reaction. Redox reactions resulting in the formation of several phases are usually not reversible, or only partially. It is also possible to perform the reactions in the solid phase through the reversible scission of the sulphur-sulphur bonds or the redox reactions involved in the transformation of the aromatic organic structures in quinonoid form, including in conjugated polymers.

The insertion materials are the electrochemical reactions active components used, in particular, in electrochemical generators, supercapacitors or light transmission modulating systems (electrochromic devices).

The progression of the ions-electrons exchange reaction requires the existence within the insertion material of a double conductivity, simultaneous with the electrons and the ions, in particular lithium ions, either one of these conductivities which may be too weak to ensure the necessary kinetic exchanges for the use of the material, in particular for electrochemical generators or supercapacitors. This problem is partly solved by using so-called “composite” electrodes, wherein the electrode material is dispersed in a matrix containing the electrolyte and a polymer binder. When the electrolyte is a polymer electrolyte or a polymer gel working in the presence of a solvent, the mechanical binding role is carried out directly by the macromolecule. Gel means a polymer matrix, solvating or not, and retaining a polar liquid and a salt, to confer to the mixture the mechanical properties of a solid while retaining at least a part of the conductivity of the polar liquid. A liquid electrolyte and the electrode material can also be maintained in contact with a small fraction of an inert polymer binder, i.e., not interacting with the solvent. With any of these means, each electrode material particle is thus surrounded by an electrolyte capable of bringing the ions in direct contact with almost the totality of the electrode material surface. To facilitate electronic exchanges, it is customary, according to the prior art, to add particles of a conductive material to one of the mixtures of the electrode material and electrolyte mentioned above. Such particles are in a very divided state. Generally, carbon-based materials are selected, and especially carbon blacks (Shawinigan or Ketjenblack®). However, the volume fractions used must be kept low because such material strongly modifies the rheology of their suspension, especially in polymers, thereby leading to an excessive porosity and loss of operating efficiency of the composite electrode, in terms of the fraction of the usable capacity as well as the kinetics, i.e., the power available. At these low concentrations used, the carbon particles structure themselves in chains, and the contact points with the electrode materials are extremely reduced. Consequently, such configuration results in a poor distribution of the electrical potential within the electroactive material. In particular, over-concentrations or depletion can appear at the triple junction points:

These excessive variations of the mobile ions local concentrations and the gradients within the electroactive materials are extremely prejudicial to the reversibility of the electrode operation over a high number of cycles. These chemical and mechanical constraints or stresses result, at the microscopic level, in the disintegration (particulation) of the electroactive material particles, a part of which become susceptible to losing contact with the carbon particles and thus becoming electrochemically inactive. The material structure can also be destroyed, with the appearance of new phases and possible release of transition metal derivatives, or other fragments in the electrolyte. These harmful phenomenons appear even more easily the larger the current density or the power requested at the electrode is.

IN THE DRAWINGS

FIG. 1 illustrates the difference between a classic electrode according to the prior art (A) and an electrode according to the invention wherein the electroactive material particles are coated with a carbonaceous coating (B).

FIGS. 2 and 3 illustrate a comparison between a sample of LiFePO₄ coated with a carbonaceous deposit, and an uncoated sample. The results were obtained by cyclic voltammetry of LiFePO₄/POE₂₀LiTFSI/Li batteries cycled at 20 mV·h⁻¹ between 3 and 3.7 V at 80° C. The first cycle is shown in FIG. 2, and the fifth in FIG. 3.

FIG. 4 illustrates the evolution of capacity during cycling for batteries containing carbonaceous and non-carbonaceous LiFePO₄ samples.

FIG. 5 illustrates the performances of a battery containing carbonaceous LiFePO₄ and cycled under an intentiostatic mode between 3 and 3.8 V at 80° C. with a charge and discharge speed corresponding to C/1.

FIG. 6 illustrates the evolution of the current vs. time of a LiFePO₄/gamma-butyrolactone LiTFSI/Li containing a carbonaceous sample and cycled at 20 mV·h⁻¹ between 3 and 3.7 V at room temperature.

FIG. 7 illustrates the evolution of the current vs. time of a LiFePO₄/POE₂₀LiTFSI/Li containing a carbonaceous sample.

FIGS. 8 and 9 illustrate a comparison between carbonaceous and non-carbonaceous LiFePO₄ samples, cycled. The results have been obtained by cyclic voltammetry of LiFePO₄/POE₂₀LiTFSI/Li batteries cycled at 20 mV·h⁻¹ between 3 and 3.7 V at 80° C. The first cycle is shown in FIG. 8, and the fifth in FIG. 9.

FIG. 10 illustrates the evolution of the capacity during cycling of batteries prepared with carbonaceous and non-carbonaceous LiFePO₄ samples.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an electrode material comprising a complex oxide corresponding to the general formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein:

A comprises an alkali metal;

M comprises at least one transition metal, and optionally at least one non-transition metal such as magnesium or aluminum; and mixtures thereof;

Z comprises at least one non-metal;

O is oxygen, N is nitrogen and F is fluorine; and

the coefficients a, m, z, o, n, f≧0 and are selected to ensure electroneutrality, wherein a conductive carbonaceous material is deposited homogeneously on a surface of the material to obtain a substantially regular electric field distribution on the surface of material particles. The similarity in ionic radii between oxygen, fluorine and nitrogen allows mutual replacement of these elements as long as electroneutrality is maintained. For simplicity, and considering that oxygen is the most frequently used element, these materials are hereinafter referred to as complex oxides. Preferred transition metals comprise iron, manganese, vanadium, titanium, molybdenum, niobium, tungsten, zinc and mixtures thereof. Preferred non-transition metals comprise magnesium and aluminum, and preferred non-metals comprise sulfur, selenium, phosphorous, arsenic, silicon, germanium, boron, and mixtures thereof.

In a preferred embodiment, the final mass concentration of the carbonaceous material varies between 0.1 and 55%, and more preferably between 0.2 and 15%.

In a further preferred embodiment, the complex oxide comprises sulfates, phosphates, silicates, oxysulfates, oxyphosphates, and oxysilicates of a transition metal and lithium, and mixtures thereof. It may also be of interest, for structural stability purposes, to partially replace the transition metal with an element having the same ionic radius but not involved in the redox process. For example, magnesium and aluminum, in concentrations preferably varying between 1 and 25% may be used.

The present invention also concerns electrochemical cells wherein at least one electrode is made of an electrode material according to the present invention. The cell can operate as a primary or secondary battery, a supercapacitor, or a light modulating system, the primary or a secondary battery being the preferred mode of operation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention allows the fabrication of electrode materials of extremely varied compositions with its surface, or most of it, coated with a uniform coating of a conductive carbonaceous material deposited chemically. The presence in the electrode materials of the invention of a uniform coating, when compared to contact points obtained with carbon powders or other prior art conductive additives, allows a regular distribution of the electrical field at the surface of the electroactive material particles. Further, the ion concentration gradients are considerably diminished. Such improved distribution of the electrochemical reaction at the surface of the particles allows on one side the maintenance of the structural integrity of the material, and on the other side improves the kinetics in terms of the current density and power availability at the electrode, because of the greater surface accessibility.

In the present application, carbonaceous material means a solid polymer comprising mainly carbon, i.e., from 60 to 100% molar, and having an electronic conductivity higher than 10⁻⁶ S/cm at room temperature, preferably higher than 10⁻⁴ S/cm. Other elements that can be present are hydrogen, oxygen, and nitrogen, as long as they do not interfere with the chemical inertia of the carbon during the electrochemical operation. The carbonaceous material can be obtained through thermal decomposition or dehydrogenation, e.g., by partial oxidation, of various organic materials. In general, any material leading, through a reaction or a sequence of reactions, to the solid carbonaceous material with the desired property without affecting the stability of the complex oxide is a suitable precursor. Preferred precursors include, but are not limited to: hydrocarbons and their derivatives, especially those comprising polycyclic aromatic moieties, like pitch and tar derivatives; perylene and its derivatives; polyhydric compounds like sugars and carbon hydrates and their derivatives; and polymers. Preferred examples of such polymers include polyolefins, polybutadienes, polyvinylic alcohol, phenol condensation products, including those from a reaction with an aldehyde, polymers derived from furfurylic alcohol, polymer derivatives of styrene, divinylbenzene, naphthalene, perylene, acrylonitrile, vinyl acetate, cellulose, starch and their esters and ethers, and mixtures thereof.

The improvement of the conductivity at the surface of the particles obtained with the carbonaceous material coating according to the present invention allows the satisfactory operation of electrodes containing electroactive materials having an insufficient electronic conductivity to obtain acceptable performances. Complex oxides with redox couples in a useful voltage range and/or using inexpensive or nontoxic elements but whose conductivity otherwise would be too low for practical use, now become useful as electrode materials when the conductive coating is present. The choice of the structures or phase mixtures possessing redox properties but having an electronic conductivity that is too low, is thus much wider than that of compounds of the prior art. It is possible to include within the redox structures, at least one element selected from non-metals (metalloids) such as sulphur, selenium, phosphorus, arsenic, silicon or germanium, wherein the greater electronegativity allows the modulation of the redox potential of the transition elements, but at the expense of the electronic conductivity. A similar effect is obtained with the partial or complete substitution of the oxygen atoms with fluorine or nitrogen.

The redox materials are described by the general formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein:

A comprises an alkali metal such as Li, Na, or K;

M comprises at least one transition metal, and optionally at least one non-transition metal such as magnesium or aluminum; or mixtures thereof;

Z comprises at least one non-metal such as S, Se, P, As, Si, Ge, B;

O is oxygen;

N is nitrogen and F is fluorine, wherein the latter elements can replace oxygen in the complex oxide because the ionic radii values for F⁻¹, O²⁻ and N³⁻ are similar; and

each coefficient a, m, z, o, n and f≧0 independently, to ensure electroneutrality of the material.

Preferred complex oxides according to the invention comprise those of formula Li_(1+x)MP_(1−x)Si_(x)O₄; Li_(1+x−y)MP_(1−x)Si_(x)O_(4−y)F_(y); Li_(3−x+z)M₂(P_(1−x−z)S_(x)Si_(z)O₄)₃; Li_(3+u−x+z)V_(2−z−w)Fe_(u)Ti_(w)(P_(1−x−z)S_(x)Si_(z)O₄)₃, or Li_(4+x)Ti₅O₁₂, Li_(4+x−2y)Mg_(y)Ti₅O₁₂, wherein w≦2; 0≦x, y≦1; z≦1 and M comprises Fe or Mn.

The carbonaceous coating can be deposited through various techniques that are an integral part of the invention. A preferred method comprises the pyrolysis of organic matter, preferably carbon-rich, in the presence of the redox material. Particularly advantageous are mesomolecules and polymers capable of easily forming, either mechanically or by impregnation from a solution or through in situ polymerization, a uniform layer at the surface of the redox material particles. A subsequent pyrolysis or dehydrogenation step thereof provides a fine and uniform layer of the carbonaceous material at the surface of the particles of the redox material. To ensure that the pyrolysis or dehydrogenation reaction will not affect the latter, it is preferred to select compositions wherein the oxygen pressure liberated from the material is sufficiently low to prevent oxidation of the carbon formed by the pyrolysis. The activity of the oxygen of compounds A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) can be controlled by the concentration of alkali metal, which itself determines the oxidation state of the transition element or elements contained in the material and being a part of the invention. Of particular interest are the compositions wherein the coefficient “a” of the alkali metal concentration allows the maintenance of the following oxidation states: Fe²⁺, Mn²⁺, V²⁺, V³⁺, Ti²⁺, Ti³⁺, Mo³⁺, Mo⁴⁺, Nb³⁺, Nb⁴⁺, W⁴⁺. Generally, oxygen pressures on the order of 10⁻²⁰ bars at 0° C. and of 10⁻¹⁰ bars at 900° C. are sufficiently low to allow the deposition of carbon by pyrolysis, the kinetics of carbon formation in the presence of hydrocarbonaceous residues resulting from the pyrolysis being quicker and less activated than oxygen formation from the redox materials. It is also possible and advantageous to select materials having an oxygen pressure in equilibrium with the materials that are inferior to that of the equilibrium C+O₂

CO₂

In this instance, the carbonaceous material can be thermodynamically stable vis-à-vis the complex oxide. The corresponding pressures are obtained according to the following equation:

${\ln\;{P\left( O_{2} \right)}} = {{\ln\;{P\left( {CO}_{2} \right)}} = \frac{94050}{R\left( {{273,2} + \theta} \right)}}$ wherein R is the perfect gas constant (1.987 cal·mole⁻¹·K⁻¹); and θ is the temperature in ° C.

Table 1 provides oxygen pressures at several temperatures:

P(O₂) P(O₂) θ (° C.) P(CO₂) = 1 atm P(CO₂) = 10⁻⁵ atm 200 3.5 × 10⁻⁴⁴ 3.5 × 10⁻⁴⁹ 300 1.4 × 10⁻³⁶ 1.4 × 10⁻⁴¹ 400 2.9 × 10⁻³¹ 2.9 × 10⁻³⁶ 500 2.5 × 10⁻²⁷ 2.5 × 10⁻³² 600 2.9 × 10⁻²⁴ 2.5 × 10⁻²⁹ 700 7.5 × 10⁻²² 7.5 × 10⁻²⁷ 800 7.0 × 10⁻²⁰ 7.0 × 10⁻²⁵ 900 3.0 × 10⁻¹⁸ 3.0 × 10⁻²³

It is also possible to perform the carbon deposition through the disproportionation of carbon oxide at temperatures lower than 800° C. according to the equation: 2CO

C+CO₂

This reaction is exothermic but slow. The complex oxide particles can be contacted with carbon monoxide, pure or diluted in an inert gas, at temperatures varying from 100 to 750° C., preferably between 300 and 650° C. Advantageously, the reaction is carried out in a fluidized bed, in order to have a large exchange surface between the gaseous phase and the solid phase. Elements and cations of transition metals present in the complex oxide are catalysts of the disproportionation reaction. It can be advantageous to add small amounts of transition metal salts, preferably iron, nickel, or cobalt, at the surface of the particles, these elements being particularly active as catalysts of the disproportionation reaction. In addition to carbon monoxide disproportionation, hydrocarbons in gaseous form can be decomposed at moderate to high temperatures to yield carbon deposits. Of special interest for the operation are the hydrocarbons with a low energy of formation, like alkenes, alkynes or aromatic rings.

In a variation, the deposition of the carbonaceous material can be performed simultaneously with a variation of the composition of alkali metal A. To do so, an organic acid or polyacid salt is mixed with the complex oxide. Another possibility comprises the in situ polymerization of a monomer or monomer mixtures. Through pyrolysis, the compound deposits a carbonaceous material film at the surface and the alkali metal A is incorporated according to the equation: A_(a)′M_(m)Z_(z)O_(o)N_(n)F_(f)+A_(a−a)′C_(c)O_(o)R′

A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) R′ being an organic radical, which may be part of a polymeric chain.

Compounds capable of permitting this reaction may include, but are not limited to, salts of carboxylic acids such as oxalic, malonic, succinic, citric, polyacrylic, polymethacrylic, benzoic, phtalic, propiolic, acetylene dicarboxylic, naphthalene di- or tetracarboxylic, perylene tetracarboxylic and diphenic acids.

Obviously, the pyrolysis of an organic material deprived of an alkali element in combination with an alkali element salt can also lead to the desired stoichiometry of the complex oxide.

It is also possible to obtain a carbonaceous material deposit, especially at low or mid-range temperatures, lower than 400° C., by reduction of carbon-halogen bonds according to the equation: CY—CY+2e⁻

—C═C—+2Y⁻ wherein Y represents a halogen or a pseudo-halogen. The term pseudo-halogen means an organic or inorganic radical capable of existing in the form of an ion Y⁻ and forming a corresponding protonated compound HY. Examples of halogen and pseudo-halogen include F, Cl, Br, I, CN, SCN, CNO, OH, N₃, RCO₂, RSO₃, wherein R is H or an organic radical. The formation by reduction of CY bonds is preferably performed in the presence of reducing elements such as hydrogen, zinc, magnesium, Ti³⁺ ions, Ti²⁺ ions, Sm²⁺ ions, Cr²⁺ ions, V²⁺ ions, tetrakis(dialkylamino ethylene) or phosphines. These reagents can optionally be obtained or regenerated electrochemically. Further, it can also be advantageous to use catalysts to increase the reduction kinetics. Palladium or nickel derivatives are particularly efficient, particularly in the form of complexes with phosphorous or nitrogen compounds like 2,2′-bipyridine. Similarly, these compounds can be generated chemically in an active form in the presence of reducing agents, such as those mentioned above, or electrochemically. Compounds capable of generating carbon by reduction include perhalocarbons, particularly in the form of polymers, hexachlorobutadiene and hexachlorocyclopentadiene.

Another way to release carbon from a low temperature process comprises the elimination of the hydrogenated compound HY, Y being as defined above, according to the equation: —CH—CY—+B

—C═C—+BHY

Compounds capable of generating carbon from reduction include organic compounds comprising an even number of hydrogen atoms and Y groups, such as hydrohalocarbons, in particular in the form of polymers, such as vinylidene polyfluoride, polychloride or polybromide, or carbon hydrates. The dehydro (pseudo) halogenation can be obtained at low temperatures, including room temperature, by reacting a base with the HY compound to form a salt. Examples of suitable bases include tertiary bases, amines, amidines, guanidines, imidazoles, inorganic bases such as alkali hydroxides, organometallic compounds behaving like strong bases, such as A(N(Si(CH₃)₃)₂, LiN[CH(CH₃)₂]₂, and butyl-lithium.

In the last two methods, it can be advantageous to anneal the material after the carbon deposition. Such treatment improves the structure or the crystallinity of the carbonaceous deposit. The treatment can be performed at a temperature varying between 100 and 1000° C., preferably between 100 and 700° C., to prevent the potential reduction of the complex oxide by the carbonaceous material.

Generally, it is possible to obtain uniform carbonaceous material coatings, ensuring a sufficient electronic conductivity, i.e., at least on the same order as the ionic conductivity of the oxide particle. The thick coatings provide a conductivity sufficient so that the binary mixture of complex oxide particles coated with the carbonaceous material, and the liquid or polymeric electrolyte or the inert macromolecular binder to be wetted with the electrolyte, is conductive by a simple contact between the particles. Generally, such behavior can be observed at volumic fractions comprised between 10 and 70%.

It can also be advantageous to select deposits of carbonaceous materials sufficiently thin to prevent obstruction of the passage of ions, while ensuring the distribution of the electrochemical potential at the surface of the particles. In this instance, the binary mixtures possibly do not possess an electronic conductivity sufficient to ensure the electronic exchanges with the electrode substrate (current collector). The addition of a third electronic conductive component, in the form of a fine powder or fibers, provides satisfactory macroscopic conductivity and improves the electronic exchanges with the electrode substrate. Carbon blacks or carbon fibers are particularly advantageous for this purpose and give satisfactory results at volumic concentrations that have little or no effect on the rheology during the use of the electrode because of the existence of electronic conductivity at the surface of the electrode material particles. Volumic fractions of 0.5 to 10% are particularly preferred. Carbon black such as Shawinigan® or Ketjenblack® are preferred. Among carbon fibers, those obtained by pyrolysis of polymers such as tar, pitch, polyacrylonitrile as well as those obtained by cracking of hydrocarbons are preferred.

Interestingly, because of its light weight and malleability, aluminium is used as the current collector constituent. This metal is nonetheless coated with an insulating oxide layer. This layer, which protects the metal from corrosion, can in certain conditions increase the thickness, leading to an increased resistance of the interface, prejudicial to the good operation of the electrochemical cell. This phenomenon can be particularly detrimental and fast when the electronic conductivity is only ensured, as in the prior art, by the carbon particles having a limited number of contact points. The use, in combination with aluminium, of electrode materials coated with a conductive carbonaceous material layer increases the exchange surface aluminium-electrode. The aluminium corrosion effects are therefore cancelled or at least significantly minimized. It is possible to use either aluminium collectors in the form of a sheet or possibly in the form of expanded or perforated metal or fibers, which allow a weight gain. Because of the properties of the materials of the invention, even in the case of expanded or perforated metal, electronic exchanges at the collector level take place without a noticeable increase of the resistance.

Whenever the current collectors are thermally stable, it is also possible to perform the pyrolysis or dehydrogenation directly on the collector so as to obtain, after carbon deposition, a continuous porous film that can be infiltrated with an ionic conductive liquid, or with a monomer or a mixture of monomers generating a polymer electrolyte after in situ polymerization. The formation of porous films in which the carbonaceous coating forms a chain is easily obtained according to the invention through pyrolysis of a complex oxide-polymer composite deposited in the form of a film on a metallic substrate.

In using the electrode material according to the invention in an electrochemical cell, preferably a primary or secondary battery, the electrolyte is preferably a polymer, solvating or not, optionally plasticized or gelled by a polar liquid in which one or more metallic salts, preferably at least a lithium salt, are dissolved. In such instance, the polymer is preferably formed from units of oxyethylene, oxypropylene, acrylonitrile, vinylidene fluoride, acrylic acid or methacrylic acid esters, or itaconic acid esters with alkyls or oxaalkyl groups. The electrolyte can also be a polar liquid immobilized in a microporous separator, such as a polyolefin, a polyester, nanoparticles of silica, alumina or lithium aluminate (LiAlO₂). Examples of polar liquids include cyclic or linear carbonates, alkyl formiates, oligoethylene glycols, α-ω alkylethers, N-methylpyrrolidinone, γ-butyrolactone, tetraalakylsulfamides and mixtures thereof.

The following examples are provided to illustrate preferred embodiments of the invention, and shall not be construed as limiting its scope.

EXAMPLE 1

This example illustrates the synthesis of a material of the present invention leading directly to an insertion material coated with a carbonaceous deposit.

The material LiFePO₄ coated with a carbonaceous deposit is prepared from vivianite (Fe₃(PO₄)₂.8H₂O) and lithium orthophosphate (Li₃PO₄) in stoichiometric amounts according to the reaction: Fe₃(PO₄)₂.8H₂O+Li₃PO₄

3 LiFePO₄

Polypropylene powder in an amount corresponding to 3% by weight of vivianite is added. The three components are intimately mixed together and ground in a zirconia ball mill. The mixture is then heated under an inert atmosphere of argon, first at 350° C. for 3 hours to dehydrate the vivianite. Subsequently, the temperature is gradually increased up to 700° C. to crystallize the material and carbonize the polypropylene. The temperature is maintained at 700° C. for 7 hours. The structure of the material obtained, as verified by X-rays, corresponds to that published for triphyllite. The amount of carbon present in the sample has been determined by elemental analysis, and gave a concentration of 0.56%. For comparison purposes, a similar sample has been prepared in similar conditions, but without the addition of polypropylene powder. This latter sample also shows a pure crystalline structure of the type LiFePO₄.

Electrochemical Properties

The materials prepared were tested in button batteries of the CR2032 type at room temperature and 80° C.

Tests at 80° C. (Polymer Electrolyte)

The materials obtained above have been tested in button batteries of the CR2032 type. The cathode was obtained by mixing together the active material powder with carbon black (Ketjenblack®) to ensure the electronic exchange with the current collector, and polyethylene oxide with a molecular weight of 400,000 is added as both a binder and an ionic conductor. The proportions, by weight, are 35:9:56. Acetonitrile is added to the mixture to dissolve the ethylene polyoxide. The mixture is homogenized and poured on a stainless steel disc of 1.7 cm². The cathode is dried under vacuum, and transferred in a Vacuum Atmospheres glove box, under helium atmosphere (<1 vpm H₂O, O₂). A sheet of lithium (27 μm) laminated on a nickel substrate is used as the anode. The polymer electrolyte comprises polyethylene oxide of weight 5,000,000 and LiTFSI (lithium bis-trifluoromethanesulfonimide) in proportions of oxygen of oxyethylene units/lithium of 20:1.

The electrochemical experiments were carried out at 80° C., the temperature at which the ionic conductivity of the electrolyte is sufficient (2×10⁻³ Scm⁻¹). The electrochemical studies are performed by slow voltammetry (20 mV·h⁻¹) controlled by a battery cycler of the Macpile® type. The batteries were charged and discharged between 3.7 and 3 V.

FIG. 2 illustrates the first cycle obtained for carbonaceous and noncarbonaceous materials prepared above. For the non-carbonaceous sample, the oxidation and reduction phenomenons extend over a wide potential range. For the carbonaceous sample, the peaks are much better defined on a narrow potential domain. The evolution of both materials during the first 5 cycles is very different (FIG. 3). For the carbon-coated sample, the oxidation and reduction kinetics become faster and faster, thus leading to better defined peaks (larger peak currents and narrower peak widths). However, for the non-carbonaceous sample, the kinetics become slower and slower. The evolution of the capacity of both samples is illustrated in FIG. 4. For the carbonaceous sample, the capacity exchanged is stable. It represents from 94 to 100% of the theoretical capacity (170 mAhg⁻¹) depending on the sample. The initial capacity of the non-carbonaceous material is around 145 mAhg⁻¹, i.e., about 85% of the theoretical capacity. For this sample, the capacity exchanged quickly decreases. After 5 cycles, the battery has lost 20% of its initial capacity.

The carbonaceous sample is cycled under an intentiostatic mode between 3.8 and 3 V with fast charging and discharging rates. The imposed currents correspond to a C/1 rate, which means that all the capacity is exchanged in 1 hour. These cycling results are shown in FIG. 5. The first 5 cycles are performed under a voltamperometric mode to activate the cathode and determine its capacity. In this instance, 100% of the theoretical capacity is exchanged during the first voltammetric cycles and 96% during the first 80 intentiostatic cycles. Subsequently, the capacity slowly decreases, and after 1000 cycles, 70% of the capacity (120 mAhg⁻¹) is still exchanged at this rate. The cycling in the potentiodynamic mode performed after 950 cycles shows that in reality, 89% of the initial capacity is still available at slower discharge rates. The loss of power is associated with the increase of the resistance at the lithium/polymer electrolyte interface. The parameter (capacity passed during charging)/(capacity passed during discharging) becomes erratic in appearance. This parameter C/D, shown on FIG. 5 at the end of cycling, leads to the presumption that dendrites are formed.

Tests at Room Temperature (Liquid Electrolyte)

The LiFePO₄ coated with a carbonaceous deposit was also tested at room temperature. In this instance, the composite cathode is prepared by mixing the active material with carbon black and EPDM (preferably dissolved in cyclohexane) in a ratio of 85:5:10. The mixture is spread onto a stainless steel current collector in the form of a disc of 1.7 cm², dried under vacuum, and kept in a glove box under helium atmosphere. As above, lithium is used as the anode. Both electrodes are separated by a Celgard™ porous membrane. The electrolyte used is a LiTFSI 0.8 molal solution in gamma-butyrolactone. The voltamperograms illustrated in FIG. 6 were obtained at room temperature under slow voltammetry (20 mV·h⁻¹) between 3 and 3.8 V. With such configuration, the oxidation and reduction kinetics appear to be much slower than at 80° C. Further, the power of the battery decreases slowly during cycling. On the other hand, the entire theoretical capacity is accessible (97.5% cycle 1, 99.4% cycle 5), i.e., reversibly exchanged without loss during cycling (5 cycles). It is not excluded that the low power of this battery may come from a poor permeation of the electrode by the electrolyte, the latter being a poor wetting agent for the binding polymer.

The example illustrates that the improvement of the material studied, because of the presence of the carbonaceous deposit at the surface of the particles, is reflected on the kinetics, the capacity and the cyclability. Further, its role is independent from that of the type of carbon black added during the preparation of composite cathodes.

EXAMPLE 2

This example shows the formation of a conductive carbonaceous deposit from a hydrocarbon gas. The synthesis described in Example 1 for the preparation of lithium iron phosphate is repeated without adding polypropylene powder, and by replacing the thermal treatment inert atmosphere with a mixture of 1% propene in nitrogen. During the thermal treatment, propene decomposes to form a carbon deposit on the material being synthesized. The resulting sample obtained contains 2.5% of carbon, as determined by chemical analysis. Cyclic voltammetry is performed on this sample under the conditions described in Example 1, and shows the important activation phenomenon during the first cycles (see FIG. 6). The improvement in redox kinetics is accompanied in this instance by an increase of the capacity reversibly exchanged. As measured during the discharge step, the initial capacity of the LiFePO₄ sample prepared represents 77% of the theoretical capacity, taking into account the 2.5% electrochemically inactive carbon. After 5 cycles, the capacity reaches 91.4%. The activation phenomenon observed is linked to the thickness of the carbon layer, which may be porous, coating the particles and capable of slowing the diffusion of the cations.

The following examples 3-5 illustrate the treatment of the complex oxide, namely the lithium iron phosphate LiFePO₄, prepared thermally and independently in order to obtain a conductive carbonaceous coating.

EXAMPLE 3

The tryphilite sample LiFePO₄ prepared above is analyzed. Its mass composition is: Fe: 34.6%, Li: 4.2%, P:19.2%, which represents a 5% difference with respect to the stoichiometry.

The powder to be treated is impregnated with an aqueous solution of commercial sucrose and dried. The amount of solution is selected to correspond to 10% of the weight of sucrose with respect to the weight of the material to be treated. Water is completely evaporated under agitation to obtain a homogeneous distribution. The use of sugar represents a preferred embodiment because it melts before being carbonized, thereby providing a good coating of the particles. Its relatively low carbon yield after pyrolysis is compensated by its low cost.

The thermal treatments are performed at 700° C. under argon atmosphere. The temperature is maintained for 3 hours. Elemental analysis shows that this product contains 1.3% by weight of carbon. Such thermal treatment leads to a black powder giving an electronic conductivity measurable with a simple commercial ohm-meter. Its electroactivity, as measured on the 1^(st) (FIG. 8) and 5^(th) (FIG. 9) charge-discharge cycle, is 155.9 mAhg⁻¹ and 149.8 mAhg⁻¹ respectively, which is 91.7% and 88.1% of the theoretical value. These values are to be compared with that of the product not coated with the carbon deposit, that has only 64% electroactivity. After 5 cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 4

Cellulose acetate is added to the phosphate LiFePO₄ of Example 3 as a precursor of the carbon coating. This polymer is known to decompose with high carbonization yields, on the order of 24%. It decomposes between 200 and 400° C. Above this temperature, the amorphous carbon rearranges to give a graphite-type structure that favors coherent and highly conductive carbon deposits.

Cellulose acetate is dissolved in acetone in a ratio corresponding to 5% by weight of the material to be treated, and dried before proceeding as above. The carbon concentration of the final product is 1.5%. The thermal treatment leads, in a similar manner, to a black powder having surface electronic conductivity. Its electroactivity, as measured on the 1^(st) (FIG. 8) and 5^(th) (FIG. 9) charge-discharge cycles, is 152.6 mAhg⁻¹ and 150.2 mAhg⁻¹ respectively, which is 89.8% and 88.3% of the theoretical value. This value is to be compared with that of the product not coated with the carbon deposit, that has only 64% electroactivity. After 5 cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 5

Perylene and its derivatives are known to lead, after pyrolysis, to graphitic-type carbons, because of the existence of condensed cycles in the starting molecule. In particular, the perylene-tetracarboxylic acid anhydride decomposes above 560° C. and provides a thin carbon layer sufficient to cover the particle surface. However, this product shows a poor solubility, and their intimate mixture with the complex oxide, here also LiFePO₄ of Example 3, is difficult to embody. To solve this problem, a polymer containing perylene groups separated with an ethylene polyoxide chain has been prepared in a first step. The oxyethylene segments are selected to be sufficiently long to act as solubilizing agents for the aromatic groups in the usual organic solvents. Therefore, commercial 3,4,9,10-perylenetetracarboxylic acid anhydride (Aldrich) is reacted with Jeffamine 600 (Hunstmann) at high temperatures, according to the following reaction:

wherein R=—[CH(CH₃)CH₂O—]_(p)(CH₂CH₂O—)_(q)[CH₂—CH(CH)₃O]_(p−1)CH₂—CH(CH)₃—1≦p≦2; 10≦n≦14

The synthesis is completed within 48 hours in dimethylacetamide under reflux (166° C.). The polymer formed is precipitated in water, and a solid-liquid separation is carried out. It is purified by dissolution in acetone, followed by re-precipitation in ether. The process allows the removal of unreacted starting materials, as well as low mass products. The powder is finally dried.

Carbonization yield of this product is on the order of 20%. The polymer is dissolved in dichloromethane in a ratio corresponding to 5% of the weight of the material to be treated before proceeding as described above in Examples 3 and 4. The carbon content of the final product is 1%. The thermal treatment leads, as described above, to a black conductive powder. Its electroactivity, as measured on the 1^(st) (FIG. 8) and 5^(th) (FIG. 9) charge-discharge cycles, is 148.6 mAhg⁻¹ and 146.9 mAhg⁻¹ respectively, which is 87.4% and 86.4% of the theoretical value. This value is to be compared with that of the product not coated with the carbon deposit, that has only 64% electroactivity. After 5 cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 6

This example illustrates the use of an elimination reaction from a polymer to form a carbonaceous deposit according to the invention.

Ferric iron sulfate (Fe₂(SO₄)₃) with a “Nasicon” orthorhombic structure was obtained from commercial hydrated iron (III) sulfate (Aldrich) by dehydration at 450° C. under vacuum. With cooling, and under stirring, the powder suspended in hexane was lithiated with stoichiometric 2 M butyl lithium to reach the composition Li_(1.5)Fe₂(SO₄)₃. 20 g of the resulting white powder were slurried in 100 mL acetone and 2.2 g of poly(vinylidene bromide) (—CH₂CBr₂)_(n)— were added and the mixture was treated for 12 hours in a ball mill with alumina balls. The suspension thus obtained was dried in a rotary evaporator and crushed as coarse powder in a mortar. The solid was treated with 3 g of diazabicyclo [5.4.0]unde-7-cene (DBU) in acetonitrile under reflux for three hours. The black powder thus obtained was filtered to eliminate the resulting amine bromide and excess reagent, rinsed with acetonitrile and dried under vacuum at 60° C. Further annealing of the carbonaceous deposit was performed under oxygen-free argon (<1 ppm) at 400° C. for three hours.

The material coated with the carbonaceous material was tested for electrochemical activity in a lithium cell with a lithium metal electrode, 1 molar lithium bis-(trifluoromethanesulfonimide) in 50:50 ethylene carbonate-dimethoxymethane mixture as electrolyte immobilized in a 25 μm microporous polypropylene separator. The cathode was obtained from the prepared redox material mixed with Ketjenblack® and slurried in a solution of ethylene-propylene-diene polymer (Aldrich), the ratio of solids content being 85:10:5. The cathode mix was spread on an expanded aluminium metal grid and pressed at 1 ton cm⁻² to a resulting thickness of 230 μm. The button cell assembly was charged (the tested material being the anode) at 1 mAcm⁻² between the cut-off potentials of 2.8 and 3.9 V. The material capacity is 120 mAhg⁻¹, corresponding to 89% of theoretical value. The average potential was obtained at 3.6 V vs. Li⁺:Li°.

EXAMPLE 7

This example illustrates the use of a nitrogen-containing compound as an electrode material.

Powdered manganous oxide MnO and lithium nitride, both commercial (Aldrich), were mixed in a dry box under helium in a 1:1 molar ratio. The reactants were put in a glassy carbon crucible and treated under oxygen-free nitrogen (<1 vpm) at 800° C. 12 g of the resulting oxynitride with an antifluorite structure Li₃MnNO were added to 0.7 g of micrometer size polyethylene powder and ball milled under helium in a polyethylene jar with dry heptane as the dispersing agent and 20 mg of Brij™ (ICI) as the surfactant. The filtered mix was then treated under a flow of oxygen-free nitrogen in a furnace at 750° C. to ensure decomposition of the polyethylene into carbon.

The carbon-coated electrode material appears as a black powder rapidly hydrolyzed in moist air. All subsequent handling was carried out in a dry box wherein a cell similar to that of Example 6 was constructed and tested for electrochemical activity of the prepared material. The electrolyte in this case is a mixture of commercial tetraethylsulfamide (Fluka) and dioxolane in a 40:60 volume ratio. Both solvents were purified by distillation over sodium hydride (under 10 torrs reduced pressure for the sulfamide). Lithium bis-(trifluoromethanesulfonimide) (LiTFSI) is added to the solvent mixture to form a 0.85 molar solution. Similar to the set-up of Example 6, the cell comprises a lithium metal electrode, the electrolyte immobilized in a 25 μm porous polypropylene separator and the material processed in a way similar to that of Example 6.

The cathode is obtained from the prepared redox material mixed with Ketjenblack® and slurried in a solution of ethylene-propylene-diene polymer, the ratio of solids content being 90:5:5. The cathode mix is pressed on an expanded copper metal grid at 1 ton cm⁻² with a resulting thickness of 125 μm. The button cell assembly is charged at 0.5 mAcm⁻² (the oxynitride being the anode) between the cut-off potentials of 0.9 and 1.8 V. The material's capacity was 370 mAhg⁻¹, i.e., 70% of the theoretical value for two electrons per formula unit. The average potential is found at 1.1 V vs. Li⁺:Li°. The material is suited for use as a negative electrode material in lithium-ion type batteries. An experimental cell of this type has been constructed with the electrode material on a copper metal grid similar to that tested previously and a positive electrode material obtained by chemical delithiation of the lithium iron phosphate of Example 1 by bromine in acetonitrile. The iron (III) phosphate obtained was pressed onto an aluminium grid to form the positive electrode and the 0.85 M LiTFSI tetraethylsulfamide/dioxolane solution used as an electrolyte. The average voltage of such cell is 2.1 V and its energy density, based on the weight of the active materials, is 240 Wh/Kg.

EXAMPLE 8

Lithium vanadium (III) phosphosilicate Li_(3.5)V₂(PO₄)_(2.5)(SiO₄)_(0.5), having a “Nasicon” structure was prepared in the following manner:

Lithium carbonate (13.85 g), lithium silicate Li₂SiO₃, (6.74 g), dihydrogen ammonium phosphate (43.2 g) and ammonium vanadate (35.1 g) were mixed with 250 mL of ethylmethylketone and treated in a ball mill with alumina balls in a thick-walled polyethylene jar for 3 days. The resulting slurry was filtered, dried and treated in a tubular furnace under a 10% hydrogen in nitrogen gas flow at 600° C. for 12 hours. After cooling, 10 g of the resulting powder were introduced in a planetary ball mill with tungsten carbide balls. The resulting powder was added to a solution of the polyaromatic polymer prepared in Example 5 (polyoxyethylene-co-perylenetetracarboxylicdimide 0.8 g in 5 mL acetone), well homogenized, and the solvent was evaporated.

The red-brown powder was thermolyzed in a stream of oxygen-free argon at 700° C. for 2 hours, leaving after cooling a black powder with a measurable surface conductivity. The material coated with the carbonaceous material was tested for electrochemical activity in a lithium-ion cell with a natural graphite electrode (NG7) coated on a copper current collector and corresponding to 24 mg/cm², 1 molar lithium hexafluorophosphate in 50:50 ethylene carbonate dimethylcarbonate mixture as electrolyte immobilized in a 25 μm microporous polypropylene separator. The cathode was obtained from the lithium vanadium phosphosilicate mixed with Ketjenblack® and slurried in a solution of vinylidenefluoride-hexafluoropropene copolymer in acetone, the ratio of solids content being 85:10:5. The cathode mix was spread on an expanded aluminium metal grid and pressed at 1 ton cm⁻² to a resulting thickness of 190 μm corresponding to an active material loading of 35 mg/cm². The button cell assembly was charged (the tested material being the anode) at 1 mAcm⁻² between the cut-off potentials of 0 and 4.1 V. The capacity of the carbon coated material was 184 mAhg⁻¹, corresponding to 78% of the theoretical value (3.5 lithium per unit formula), slowly fading with cycling. In a comparative test, a similar cell constructed using the uncoated material, as obtained after milling the heat treated inorganic precursor but omitting the addition of the perylene polymer, shows a capacity of 105 mAhg⁻¹, rapidly fading with cycling.

EXAMPLE 9

This example illustrates the formation of a carbonaceous coating simultaneous to a variation of the alkali metal content of the redox material.

13.54 g of commercial iron (III) fluoride (Aldrich), 1.8 g of the lithium salt of hexa-2,4-dyine dicarboxylic acid are ball milled in a thick-walled polyethylene jar with alumina balls, in the presence of 100 mL of acetonitrile. After 12 hours, the resulting slurry was filtered and the dried powder was treated under a stream of dry, oxygen-free nitrogen in a tubular furnace at 700° C. for three hours. The resulting black powder contained from elemental analysis: Fe: 47%, F: 46%, Li: 1.18%, C: 3.5%, corresponding to the formula Li_(0.2)FeF₃C_(0.35). The electrode material was tested for its capacity in a cell similar to that of Example 6 with the difference being that the cell is first tested on discharge (the electrode material as cathode), and then recharged. The cut-off voltages were chosen between 2.8 and 3.7 V. The experimental capacity on the first cycle was 190 mAhg⁻¹, corresponding to 83% of the theoretical value. For comparison, a cell with FeF₃ as the electrode material and no carbonaceous coating has a theoretical capacity of 246 mAhg⁻¹. In practice, the first discharge cycle obtained in similar conditions to the material of the invention is 137 mAhg⁻¹.

EXAMPLE 10

This example also illustrates the formation of a carbonaceous coating simultaneous to a variation of the alkali metal content of the redox material.

Commercial polyacrylic acid of molecular weight 15,000 was dissolved as 10% solution in water/methanol mixture and titrated with lithium hydroxide to a pH of 7. 4 μL of this solution were dried in the crucible of a thermogravimetry air at 80° C. to evaporate the water/methanol. The heating was then continued to 500° C., showing a residue of 0.1895 mg of calcination residue as lithium carbonate.

18.68 g of commercial iron (III) phosphate dihydrate, (Aldrich), 8.15 g lithium oxalate (Aldrich), 39 mL of the lithium polyacrylate solution, 80 mL of acetone and 40 mL of 2,2-dimethoxy acetone as water scavenger were ball milled in a thick-walled polyethylene jar with alumina balls. After 24 hours, the resulting slurry was filtered and dried. The resulting powder was treated under a stream of dry, oxygen-free nitrogen in a tubular furnace at 700° C. for three hours, resulting in a blackish powder. The resulting product had the following elemental analysis: Fe: 34%, P: 18.8%, Li: 4.4%, C: 3.2%. The X-ray analysis confirmed the existence of pure triphilite LiFePO₄ as the sole crystalline component. The electrode material was tested for its capacity in a cell similar to that of Example 1 with a PEO electrolyte, and then recharged. The cut-off voltages were chosen between 2.8 and 3.7 V. The experimental capacity on the first cycle was 135 mAhg⁻¹, corresponding to 77% of the theoretical value, increasing to 156 mAhg⁻¹ (89%) while the peak definition improved with further cycling. 80% of this capacity is accessible in the potential range 3.3-3.6 V vs. Li⁺:Li°.

EXAMPLE 11

The compound LiCo_(0.75)Mn_(0.25)PO₄ was prepared from intimately ground cobalt oxalate dihydrate, manganese oxalate dihydrate and dihydrogen ammonium phosphate by firing in air at 850° C. for 10 hours. The resulting mauve powder was ball milled in a planetary mill with tungsten carbide balls to an average grain size of 4 μm. 10 g of this complex phosphate were triturated in a mortar with 10 mL of 6% solution of the perylene polymer of Example 5 in methyl formate. The solvent rapidly evaporated. The resulting powder was treated under a stream of dry, oxygen-free argon in a tubular furnace at 740° C. for three hours, resulting in a black powder. The electrochemical activity of the cell was tested in a lithium-ion cell similar to that of Example 6. The electrolyte was, in this case, lithium bis-fluoromethanesulfonimide (Li[FSO₂]₂N) dissolved at a concentration of 1 M in the oxidation-resistant solvent dimethylamino-trifluoroethyl sulfamate (CF₃CH₂OSO₂N(CH₃)₂). When initially charged, the cell showed a capacity of 145 mAhg⁻¹ in the voltage window 4.2-4.95 V vs. Li⁺:Li°. The battery could be cycled for 50 deep charge-discharge cycles with less than 10% decline in capacity, showing the resistance of the electrolyte to high potentials.

EXAMPLE 12

The compound Li₂MnSiO₄ was prepared by calcining the gel resulting from the action of a stoichiometric mixture of lithium acetate dihydrate, manganese acetate tetrahydrate and tetraethoxysilane in a 80:20 ethanol water mixture. The gel was dried in an oven at 80° C. for 48 hours, powdered and calcined under air at 800° C. 3.28 g of the resulting silicate and 12.62 g of lithium iron phosphate from Example 3 were ball milled in a planetary mill similar to that of Example 11, and the powder was treated at 800° C. under a stream of dry, oxygen-free argon in a tubular furnace at 740° C. for 6 hours. The complex oxide obtained after cooling has the formula Li_(1.2)Fe_(0.8)Mn_(0.2)P_(0.8)Si_(0.2)O₄. The powder was moistened with 3 mL of a 2% solution of cobalt acetate, then dried. The powder was treated in the same tubular furnace at 500° C. under a flow of 1 mL/s of 10% carbon monoxide in nitrogen for two hours. After cooling, the resulting black powder was tested for electrochemical activity in conditions similar to those of Example 1. With a PEO electrolyte at 80° C., the capacity was measured from the cyclic voltamogram curve at 185 mAhg⁻¹ (88% of theory) between the cut-off voltages of 2.8 and 3.9 V vs. Li⁺:Li°. The uncoated material tested in similar conditions, has a specific capacity of 105 mAhg⁻¹.

EXAMPLE 13

Under argon, 3 g of lithium iron phosphate from Example 3 was suspended in 50 mL acetonitrile to which was added 0.5 g of hexachlorocyclopentadiene and 10 mg of tetrakis(triphenylphosphine)nickel (0). Under vigorous stirring, 1.4 mL of tetrakis(dimethylamino)ethylene was added dropwise at room temperature. The solution turned blue, and after more reducing agent was added, black. The reaction was left under stirring for 24 hours after completion of the addition. The resulting black precipitate was filtered, washed with ethanol and dried under vacuum. Annealing of the carbon deposit was performed at 400° C. under a flow of oxygen-free gas for 3 hours. The resulting black powder was tested for electrochemical activity in conditions similar to those of Example 1. The measured capacity between the cut-off voltages of 2.9 and 3.7 V vs. Li⁺:Li° was found experimentally at 160 mAhg⁻¹ (91% of theory). The uncoated material has a specific capacity of 112 mAhg⁻¹ in the same experimental conditions.

EXAMPLE 14

The spinel compound Li_(3.5)Mg_(0.5)Ti₄O₁₂ was prepared by sol-gel technique using titanium tetra(isopropoxide) (28.42 g), lithium acetate dihydrate (35.7 g) and magnesium acetate tetrahydrate (10.7 g) in 300 mL 80:20 isopropanol-water. The resulting white gel was dried in an oven at 80° C. and calcined at 800° C. in air for 3 hours, then under 10% hydrogen in argon at 850° C. for 5 hours. 10 g of the resulting blue powder were mixed with 12 mL of a 13 wt % solution of the cellulose acetate in acetone. The paste was dried and the polymer carbonized in the conditions of Example 4 under inert atmosphere at 700° C.

The positive electrode of an electrochemical super capacitor was built in the following manner. 5 g of carbon-coated LiFePO₄ from Example 3, 5 g of Norit®, activated carbon, 4 g of graphite powder (2 μm diameter), 3 g of chopped aluminium fibers (20 μm long and 5 mm diameter), 9 g of anthracene powder (10 μm) as a pore former and 6 g of polyacrylonitrile were mixed in dimethylformamide wherein the polymer dissolved. The slurry was homogenized and coated onto an aluminium foil (25 μm) and the solvent was evaporated. The coating was then slowly brought to 380° C. under nitrogen atmosphere. The anthracene evaporated to leave a homogeneous porosity in the material and the acrylonitrile underwent thermal cyclization to a conductive polymer consisting of fused pyridine rings. The thickness of the resulting layer is 75 μm.

A similar coating is done for the negative electrode with a slurry where LiFePO₄ is replaced with the coated spinel as prepared above. The super capacitor assembly is obtained by placing the two prepared electrodes face to face, separated by a 10 μm-thick polypropylene separator soaked in 1 molar LiTFSI in acetonitrile/dimethoxyethane mixture (50:50). The device can be charged at 30 mAcm⁻² and 2.5 V and delivers a specific power of 3 kW/L⁻¹ at 1.8 V.

EXAMPLE 15

A light modulating device (electrochromic window) was constructed in the following manner.

LiFePO₄ from Example 3 was ball milled in a high energy mill to particles of an average size of 120 nm. 2 g of the powder were mixed with 1 mL of a 2 wt % solution of the perylene-co-polyoxyethylene polymer of Example 5 in methyl formate. The paste was triturated to ensure uniform distribution of the polymer at the surface of the particles, and the solvent was evaporated. The dry powder was treated under a stream of dry, oxygen-free nitrogen in a tubular furnace at 700° C. for three hours to yield a light gray powder.

1 g of the carbon-coated powder was slurried in a solution of 1.2 g polyethyleneoxide-co-(2-methylene)propane-1,3-diyl prepared according to J. Electrochem. Soc., 1994, 141(7), 1915 with ethylene oxide segments of molecular weight 1000, 280 mg of LiTFSI and 15 mg of diphenylbenzyl dimethyl acetal as photoinitiator in 10 mL of acetonitrile. The solution was coated using the doctor blade process onto an indium-tin oxide (ITO) covered glass (20 S⁻¹□) to a thickness of 8 μm. After evaporation of the solvent, the polymer was cured with a 254 nm. UV light (200 mWcm⁻²) for 3 minutes.

Tungsten trioxide was deposited by thermal evaporation onto another ITO covered glass to a thickness of 340 nm. The device assembly was done by applying a layer of a polyethylene oxide (120 μm) electrolyte with LiTFSI in an oxygen (polymer) to salt ratio of 12, previously coated on a polypropylene foil and applied to the WO₃-coated electrode using the adhesive transfer technology. The two glass electrodes were pressed together to form the electrochemical chain: glass/ITO/WO₃/PEO-LiTFSI/LiFePO₄ composite electrode/ITO/glass

The device turned blue in 30 seconds upon application of a voltage (1.5 V, LiFePO₄ side being the anode) and bleached on reversal of the voltage. The light transmission is modulated from 85% (bleached state) to 20% (colored state).

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications, and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present description as come within known or customary practice within the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims. 

1. An alkali-metal ion-reversible electrode material comprising an alkali-based complex oxide of the general formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein: A comprises an alkali metal selected from the group consisting of lithium, sodium, and potassium; M comprises at least one transition metal, and optionally at least one non-transition metal, or mixtures thereof; Z comprises at least one non-metal; O is oxygen; N is nitrogen; and F is fluorine; and wherein said alkali-based complex oxide comprises a conductive carbon coating in a non-powdery form on a surface thereof, said conductive carbon coating being effective to obtain a regular electric field distribution on the surface of said alkali-based complex oxide; and wherein the coefficients a, m, z, o, n, and f>0 and are selected to ensure electroneutrality, and to avoid oxidation of said conductive carbon coating during deposition thereof on said surface.
 2. The material according to claim 1, wherein said conductive carbon coating is obtained by pyrolysis of a carbonaceous precursor.
 3. The material according to claim 2, wherein said carbonaceous precursor is selected from the group consisting of hydrocarbons and their derivatives; perylene and its derivatives; polyhydric compounds and their derivatives; a polymer or a mixture of polymers; and mixtures thereof.
 4. The material according to claim 2, wherein said carbonaceous precursor comprises a polymer.
 5. The material according to claim 4, wherein said polymer is selected from the group consisting of polyolefins, polybutadienes, polyvinylic alcohol, phenol condensation products, polymers derived from furfurylic alcohol, polymer derivatives of styrene, divinylbenzene, acrylonitrile, vinyl acetate, cellulose, starch and its esters, and mixtures thereof.
 6. The material according to claim 2, wherein said carbonaceous precursor is in gaseous form and is selected from the group consisting of gaseous hydrocarbons, CO, CO precursors and mixtures thereof.
 7. The material according to claim 6, wherein said carbonaceous precursor in gaseous form comprises hydrogen.
 8. The material according to claim 6, wherein said carbonaceous precursor in gaseous form is formed in situ during pyrolysis.
 9. The material according to claim 2, wherein said carbonaceous precursor comprises CO and said conductive carbon coating is obtained as a result of the equilibrium reaction: 2CO⇄CO₂+C.
 10. The material according to claim 9, wherein said carbonaceous precursor is formed in situ during pyrolysis.
 11. The material according to claim 1, wherein the transition metal is selected from the group consisting of iron, manganese, vanadium, titanium, molybdenum, niobium, tungsten, zinc and mixtures thereof; the non-transition metal is selected from the group consisting of magnesium and aluminum; and the non-metal is selected from the group consisting of sulfur, selenium, phosphorous, arsenic, silicon, germanium, boron, and mixtures thereof.
 12. The material according to claim 11, wherein the transition metal is selected from the group consisting of Fe²⁺, Mn²⁺, V²⁺, V³⁺, Ti²⁺, Ti³⁺, Mo³⁺, Mo⁴⁺, Nb³⁺, Nb⁴⁺ and W⁴⁺.
 13. The material according to claim 1, wherein a final mass concentration of said conductive carbon coating is between 0.1 and 55%.
 14. The material according to claim 1, wherein the Z_(z)O_(o) of said alkali-based complex oxide is selected from the group consisting of sulfates, phosphates, silicates, oxysulfates, oxyphosphates, oxysilicates, and mixtures thereof; and wherein the alkali metal comprises lithium.
 15. The material according to claim 1, wherein said alkali-based complex oxide is of the general formula Li_(1+x) MP_(1−x)Si_(x)O₄ or Li_(1+x+y)MP_(1−x)Si_(x)O_(4−y)F_(y) wherein 0≦x, y≦1 and M comprises Fe or Mn.
 16. The material according to claim 1, wherein said alkali-based complex oxide is of general formula Li_(3−x+2) M₂ (P_(1−x−z)S_(x)Si_(z) ^(O) ₄)₃ wherein M comprises Fe or Mn, 0.1≦x and z≦1.
 17. The material according to claim 1, wherein said alkali-based complex oxide is of general formula Li_(3+y−x+z)V_(2−z−w)Fe_(y)Ti_(w) (P_(1−x−z)S_(x)Si_(z)O₄)₃, Li_(4+x)Ti₅O₁₂, or Li_(4+−2y)Mg_(y)Ti₅O₁₂, wherein x≧0; w≦2; y≦1 and z≦1.
 18. The material according to claim 1, wherein the value of coefficient “a” is selected such that the pressure of oxygen in equilibrium with said conductive carbon coating is on the order of 10⁻²⁰ bars at 0° C. and 10⁻¹⁰ bars at 900° C.
 19. The material according to claim 18, wherein the pressure of oxygen in equilibrium with said conductive carbon coating is lower than the pressure of oxygen corresponding to the equilibrium reaction C+O₂⇄CO₂ in the temperature range between 100 and 900° C. during preparation of the material.
 20. The material according to claim 18, wherein the pressure of oxygen in equilibrium with said conductive carbon coating is lower than the pressure of oxygen corresponding to the equilibrium reaction C+O₂⇄CO₂ in the temperature range between 100 and 750° C. during preparation of the material.
 21. The material according to claim 20, wherein the pressure of oxygen of the gaseous phase in contact with said conductive carbon coating is imposed by the equilibrium reaction C+O₂⇄CO₂ in the temperature range between 100 and 900° C. during preparation of the material.
 22. The material according to claim 20, wherein the pressure of oxygen of the gaseous phase in contact with said conductive carbon coating is imposed by the equilibrium reaction C+O₂⇄CO₂ in the temperature range between 100 and 750° C. during preparation of the material.
 23. The material according to claim 1, wherein the material comprises one of (i) a phosphate and (ii) a silicophosphate including the tryphilite structure, corresponding to the formula Li_(1+x)MP_(1−x)Si_(x)O₄ or Li_(1−x−y)MP_(1−x)Si_(x)O_(4−y)F_(y) wherein 0≦x, y≦1, and M comprises Fe or Mn.
 24. The material according to claim 1, wherein the material comprises one of (i) a phosphate and (ii) a silicophosphate including the tryphilite structure, corresponding to the formula Li_(3−x+z)M₂(P_(1−x−z)S_(x)Si_(z)O₄)₃ wherein M comprises Fe or Mn, 0≦x, and z≦1.
 25. The material according to claim 1, wherein the material comprises an alkali-based complex oxide, corresponding to the formula Li_(3+y−x+z)V_(2−z−w)Fe_(y)Ti_(w)(P_(1−x−z)S_(x)Si_(z)O₄)₃, Li_(4+x)Ti₅O₁₂, or Li_(4+x−2y)Mg_(y) Ti₅O₁₂, wherein x≧0; w≦2; y<1 and z≦1.
 26. The material according to claim 1, wherein said conductive carbon coating is structurally integral.
 27. The material according to claim 1, wherein said conductive carbon coating partly covers the surface of said alkali-based complex oxide, thereby allowing said regular electric field distribution.
 28. The material according to claim 1, wherein said conductive carbon coating is sufficiently thin to allow passage of ions therethrough while ensuring said regular electric field distribution on the surface of said alkali-based complex oxide.
 29. The material according to claim 1, wherein said conductive carbon coating is substantially uniform.
 30. An electrode material comprising an alkali-based complex oxide of the general formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein: A comprises an alkali metal selected from the group consisting of lithium, sodium, and potassium; M comprises at least one transition metal, and optionally at least one non-transition metal, or mixtures thereof; Z comprises at least one non-metal; O is oxygen; N is nitrogen; and F is fluorine; and wherein said alkali-based complex oxide comprises a conductive carbon coating in a non-powdery form on a surface thereof, said conductive carbon coating being effective to obtain a regular electric field distribution on the surface of said alkali-based complex oxide; wherein a≧0.1; m, o and z>0; and n and f≧0 and are selected to ensure electroneutrality in said alkali-based complex oxide and to avoid oxidation of said conductive carbon coating during deposition thereof on said surface, prepared by a) providing the alkali-based complex oxide defined above, wherein Z and O are such that said alkali-based complex oxide is selected from the group consisting of sulfates, phosphates, silicates, oxysulfates, oxyphosphates, oxysilicates and mixtures thereof; wherein said alkali-based complex oxide stabilizes all or part of the transition metal in its lower oxidation level, not considering 0, and conducts alkali metal ions; and b) contacting said alkali-based complex oxide with a carbonaceous precursor under conditions of temperature, time and pressure effective to form said conductive carbon coating on said alkali-based complex oxide, said carbon coating being substantially homogeneous and covering all or most of the surface with a thin film of carbon that is optimized to give electronic conductivity in said alkali-based complex oxide, without blocking alkali-ion exchanges at the surface of said alkali-based complex oxide and without reducing the transition metal of said alkali-based complex oxide to a metallic state, the alkali-based complex oxide having mixed electron and alkali-ion conductivity and alkali-ion-exchange capability for use in a battery.
 31. The material according to claim 30, wherein said alkali-based complex oxide is provided by pyrolysing at least one alkali-based complex oxide precursor under conditions effective to form said alkali-based complex oxide.
 32. The material according to claim 31, wherein pyrolysing the at least one alkali-based complex oxide precursor is carried out simultaneously with the formation of the conductive carbon coating on said alkali-based complex oxide.
 33. The material according to claim 31, wherein said pyrolysing step further comprises the step of dispersing a polymer or polymer mixture with said alkali-based complex oxide or with precursors of said alkali-based complex oxide so as to form a dispersion, and pyrolysing the dispersion.
 34. The material according to claim 31, wherein upon said pyrolysing step, said carbonaceous precursor provides CO effective to form said conductive carbon coating according to the equilibrium reaction: 2CO⇄CO₂+C.
 35. The material according to claim 30, wherein said contacting step is carried out by pyrolysing said carbonaceous precursor in the presence of said alkali-based complex oxide or in the presence of alkali-based complex oxide precursors.
 36. The material according to claim 35, wherein said pyrolysing step is carried out under vacuum.
 37. The material according to claim 35, wherein said pyrolysing step is carried out under non-oxidizing conditions.
 38. The material according to claim 35, wherein said pyrolysing step is carried out under reducing conditions.
 39. The material according to claim 35, wherein said carbonaceous precursor is in a form selected from the group consisting of solid, liquid, and gas.
 40. The material according to claim 39, wherein said carbonaceous precursor is in solid form and is selected from the group consisting of a polymer, a solid hydrocarbon, and derivatives and mixtures thereof.
 41. The material according to claim 39, wherein said carbonaceous precursor is in solid form and comprises a polymer or a mixture of polymers.
 42. The material according to claim 41, wherein said polymer is selected from the group consisting at polyolefins, polybutadienes, polyvinylic alcohol, phenol condensation products, polymers derived from furfurylic alcohol, polymers derivatives of styrene, divinylbenzene, acrylonitrile, vinyl acetate, cellulose, starch and its esters, and mixtures thereof.
 43. The material according to claim 39, wherein said carbonaceous precursor material is in liquid form and is selected from the group consisting of a liquid solution of a polymer, a liquid monomer, a liquid hydrocarbon, and mixtures and derivatives thereof.
 44. The material according to claim 39, wherein said carbonaceous precursor is in gaseous form and is selected from the group consisting of gaseous hydrocarbons, CO, CO precursors, and mixtures thereof.
 45. The material according to claim 44, wherein said carbonaceous precursor in gaseous form includes hydrogen.
 46. The material according to claim 44, wherein said carbonaceous precursor in gaseous form is formed in situ during pyrolysis.
 47. The material according to claim 44, wherein said carbonaceous precursor comprises CO and wherein conditions are effective to form said conductive carbon coating according to the equilibrium reaction: 2CO⇄CO₂+C.
 48. The material according to claim 35, wherein said pyrolysing step is carried out at a temperature lower than 900° C.
 49. The material according to claim 30, wherein said contacting step is carried out by pyrolysing said carbonaceous precursor in the presence of alkali-based complex oxide precursors. 